19.7.2007 slide 1

Polar Ozone: Past and presentChapter 4 of WMO 2006 Ozone Assessment

Summary

Part 1 Polar stratospheric observations update

Part 2 Progress in our understanding of physical and

chemical processes

Mark Weber2007/07/19

19.7.2007 slide 2

Part 1Polar stratospheric observations update

19.7.2007 slide 3

Executive Summary: Arctic observations

Arctic spring total ozone over the last decade were lower than in the eighties

19.7.2007 slide 4

Executive Summary: Arctic observations

Antthropogenic chemical loss and variability and transport are equally important for year-to-year Arctic ozone variability

transport

chemicalloss

19.7.2007 slide 5

Executive Summary: Arctic observations

Spring Arctic ozone is highly variable depending on dynamical conditions, variability in polar temperatures and ozone transport are dynamically driven

Eddy heat flux is a measure of planetary wave activity

19.7.2007 slide 6

Executive Summary: Arctic observations

Spring Arctic ozone is highly variable depending on dynamical conditions, variability in polar temperatures and ozone transport are dynamically driven

Fig. 4-5, 4-1, 4-4

19.7.2007 slide 7

Executive Summary: Arctic observations

For coldest Arctic winter, the volume of air with temperatures low enough for PSC formation has increased significantly since the 1960s

19.7.2007 slide 8

Executive Summary: Arctic observations

Column ozone loss in Arctic winter 2004/05 was among the largest ever observed, with particular high losses below 18 km

Observed chemical ozone loss from satellites (HALOE, ACE), ozone sondes, and groundbased DOAS (SAOZ)

19.7.2007 slide 9

Executive Summary: Arctic observations Column ozone loss in Arctic winter

2004/05 was among the largest ever observed, with particular high losses below 18 km

Vortex averaged ozone loss from satellites (MLS, SAGE II, POAM III) and ozonesondes

Note difference in altitudes of maximum loss from year-to-year (2000 vs. 2005)

19.7.2007 slide 10

Executive Summary: Antarctic observations

Antarctic ozone depletion has stabilized during the last decade (1995-2005)

Explanations:Saturation of ozone

loss (complete loss between 15 and 20 km altitudes)

Higher levels of dynamical forcing (see 2002 ozone hole anomaly)

<220 DU

19.7.2007 slide 11

Executive Summary: General

Large inter-annual variability in polar stratospheric temperatures complicate the interpretation of trends

19.7.2007 slide 12

Part 2 Progress in our understanding of physical and chemical

processes

19.7.2007 slide 13

Current understanding of physical and chemical processes

Chlorine cycle: Chlorine activation on PSCs (<195

K) Photolysis rates of ClOOCl is highly

uncertain (absorption cross-section) Uncertainty of the relative role of

ClOOCl+M2ClO+M (~10%, null cycle) vs. Reaction (1b)

Bromine cycle BrO abundance is not dependent on

PSC existence About half of the polar ozone loss

may be due to the bromine cycle total inorganic bromine (Bry, source

of BrO) is highly uncertain due to uncertainties in the very-short-lived (VSL) bromo-carbons entering the stratosphere

1

2 1

3 2 1

3 2

( )2 ( )

2 ( ): 2 3

a

b

c

ClO ClO M ClOOCl M kClOOCl h Cl O JCl O ClO O k

Net O O

2 2

3 2 2

3 2 2

3 2

( )( )

( ): 2 3

a

b

c

BrO ClO h Br Cl O JBr O BrO O kCl O ClO O kNet O O

loss cycle 1:

loss cycle 2:

19.7.2007 slide 14

Chlorine cycle

Best agreement with ClOx observations using photolysis rates (absorption cross-sections) from Burkholder et al. 1990

[ ] / [ ][ ] /[ ]

OBS

MODEL

ClOOCl ClOClOOCl ClO

J1b

k1a

1

2 1

3 2 1

3 2

( )2 ( )

2 ( ): 2 3

a

b

c

ClO ClO M ClOOCl M kClOOCl h Cl O JCl O ClO O k

Net O O

19.7.2007 slide 15

Bromine cycle

Measured BrO ( about 20 ppt) is higher than modelled assuming methyle bromide (CH3Br) and halons as primary source

Contribution from VSLS?

modelled BrO from CH3Br and halons

BrO from ballon measurements

19.7.2007 slide 16

Bromine cycle

Modelled ClOx (blue solid line)(blue solid line) in best agreement with observations (grey symbols) using Burkholder J1a rates (ClOOCl photolysis) and measured BrO

Contribution from bromine cycle to chemical loss is on the order of 30-50%

Arctic winter 1999/2000

19.7.2007 slide 17

Arctic O3 column loss and PSC volume

Color: Observation from HALOE (circles) and ozone sondes (squares) Black: CTM model results using lower grid resolution (light) and higher grid resolution, improved denitrification/ sedimentation, and updated reaction rates (solid)

Modelled polar ozone loss is in better agreement with observation/ measurements

This linear relationship is an useful diagnostics for climate models

19.7.2007 slide 18

Summary

Although the forcing of polar temperatures and vortex strength from planetary and gravity waves are well established, the causes of forcing variability (on dcadal scale) are still unknown

Cold Arctic winters have gotten colder over four decades, the temperature change exceeds that expected from changes in greenhouse gases, the reason for this is still unknown

BrO may play a larger role in polar ozone loss (up to 50%), but there is still large uncertainty in the polar bromine budget.

The exact NAT nucleation mechanism in PSCs is still not completely understood, although progress has been made in parameterising denitrification in models.